It is often desired to measure the tensional strength or stress in a pierce-able material such as in sediment or soil. (Note that the term “tension” is utilized herein in the engineering sense of a stress that pulls on both ends of a member and not in the sense of the tenacity with which soil particles hold to water.)
The strength of soil, snow, sediment and other soft materials is a measure of the capacity of the material to resist deformation and can be understood in terms of the amount of energy required to break apart pieces of the material or move implements through the material or a measure of the amount of weight a given area of the material will support. Material failure may be in the form of permanent deformation through externally applied stress, e.g., sinking of a structure into the soil, breakup of the soil surface as in plowing; or alternatively failure may be from stresses affecting an unstable slope as in avalanches, mudslides, or erosion.
Soil strength tests are well established and described in multiple standard tests such as ASTM D1 194 (load plates), D1586 (standard penetration test), D3441 (cone penetration test), D4429 (bearing ratio in place) and ASAE S313.2 (soil cone penetrometer). All of these tests pertain to measurements made by compressing the test material. Similarly, testing of soils using a flat plate dilatometer for determining stress/strain characteristics (ASTM 6635-01) is also done using compression. A less common test for measuring the strength of soil determines shear strength as covered in ASTM D2573-08, Standard Test Method for Field Vane Shear Test in Cohesive Soil, or additionally ASTM STP 1014.
While measurements of compression and shear of soil, sediment, snow, food stuffs and other such pierce-able materials give important information about strength, there is additional information in measurements made with the sample in tension. In particular, the strength of materials in engineering studies is known to show differences depending on whether the test sample is subjected to compression or tension. For example, fibre reinforced materials typically show greater strengths in tension than in compression, and fibrous materials in soils and sediment are common. In addition, where a material contains defects, e.g., small cracks, tension can result in failure by fracture, whereas, compression may force small defects to close and not act as loci of failure.
Many studies of the strength of soil, snow, sediment, food stuffs and other such pierce-able materials have shown the importance of fracture as a mechanism of failure, e.g., failure of sediments during methane bubble growth and rise (see from reference list below Johnson et al., 2002; Boudreau et al., 2005); failure of sediments during animal locomotion (Dorgan, et al. (2005) and Jumars et al. (2007)); failure of soils (e.g., Wang, et al., 2007; Hallet and Newson, 2001); failure of snow (e.g., McClung, 2007); failure of foodstuff (e.g., Scanlon and Long, 1995). However, probes for measuring the strength of soil, snow, sediment, foodstuffs and other such pierce-able material have measured compression or shear strength, and laboratory measurements have typically relied on engineering type sample compression or tension loading or three point bending or cantilever tests. In our understanding, there are at present no in situ probes for measuring failure of soil, snow, sediment, foodstuff and other such pierce-able materials in tension.
In situ probe measurements can provide information on material strength at small intervals of distance, whereas typical engineering measurements on samples in tension or compression cause the sample to fail only at its weakest point which provides only a single datum for that sample. While in situ probes offer advantages in resolution of material strength over distance, current in situ probes typically measure compression or shear failure. This is a problem because the strength of sediment, soil, snow, foodstuff or other such pierce-able materials in tension is important for identifying discontinuities or other regions of weakness that may result in slumping or failure as in mud slides and avalanches or may indicate regions of weakness that may result in erosion or other modes of failure. Measurement of materials in tension is superior to measurement in compression for identifying dislocations, defects, and weak layers since compression presses surfaces together rather than pulling them apart.
Further, measurements of failure in tension provide different information than failure in shear because shear strength can be enhanced through interlocking of grains of sand or gravel, or granular snow or ice. Shear may actually close defects that tension will open and cause material failure. For example, measurements of compression and shear failure on clean sand show a much greater strength than measurements of tension on the same material. The difficulty of interpreting measurements from a compressive type probe are apparent in use of the cone penetrometer, a type of probe often used to determine strength of soil and sediments. This device uses a cone shaped probe head that is driven into the soil either at constant speed or with constant force and the resistance to penetration is measured with a force sensor. Considerable effort has gone into improving this method by adding sensors to measure friction force and pore water pressure. Still, difficulty in interpreting cone penetrometer measurements in terms of type of material, e.g., sand, silt, gravel, etc., requires typically that samples of the material be collected and assessed.
In determining the strength of snow to assess the risk of avalanches methods are often simple and effective, but do not provide information on material strength other than failure under the conditions of the test. This means that while a particular test may show the snow to be safe, there is not sufficient information to determine if small changes in conditions, e.g., moisture content, temperature, etc., might make the snow pack prone to failure. Commonly used methods for measuring stability of snow against avalanches typically involve digging snow pits and then determining the stability when applying stress at the surface. One example is the stuffblock test (Birkeland, K. W., R. F. Johnson and D. Herzberg. 1996). In this test a bag filled with 10 lbs of snow is dropped from various heights onto a column of snow at the edge of a snow pit. In application of this method the snow fails typically at a single point, whereas, measurements with an in situ probe that measures failure under tension could provide measurements of material strength over small depth intervals and thereby identify regions that may be near failure and that may fail if the conditions change.
Force measurements in food assessments typically involve: puncture, compression-extrusion, cutting-shear, compression, tension, torsion, bending and snapping and deformation. Tension measurements are typically done with samples of specific dimensions subjected to typical engineering testing to determine elongation and failure. Probes used for assessing foods are typically for measurement of puncture strength, moisture or thermal properties.
Mitsuru Taniwaki, et al., developed a method for assessing food texture in which a probe is inserted into a food sample and the vibration caused by the sample's fracture is detected using a piezoelectric sensor. The method follows previous work in which the sounds of food mastication were recorded. Results show promise for assessing food texture, but have not proven useful for quantifying fracture strength.
A variety of probes are disclosed in the patent literature. U.S. Pat. No. 4,806,153 discloses a penetrometer for soils that uses sensors to measure compressive resistance to penetration, friction from penetration and pore water pressure. U.S. Pat. No. 5,831,161 discloses a penetrometer for snow that measures compressive resistance to penetration using a force transducer. U.S. Pat. No. 7,040,146 discloses a soil and snow penetrometer that uses sensors to measure the compressive resistance to penetration of a probe head into soil and snow. It uses a load cell and accelerometer and a processor to interpret results in terms of the compressive vertical strength of soil or snow. U.S. Pat. Nos. 4,061,021, 5,726,349, 5,663,649 also describe probes that measure compressive strength of soils and other soft materials.
In addition, probes have been described for measuring shear strength and Young's modulus of soils, snow and sediments, including U.S. Pat. No. 4,594,899 which describes a probe for soil that is comprised of two concentric cylinders. When inserted into the soil, the rotary response of the inner cylinder is measured in response to a known rotary excitation and is interpreted in terms of the soil liquefaction resistance and soil degradation.
Despite the considerable art in the field, a need still exists for an in situ method and apparatus for measuring the tensional strength of soil, snow, sediment, foodstuff and other such pierce-able materials.